1. Field of the Invention
The present invention relates to a light-emitting element (also referred to as an EL element) including an electroluminescence layer; a light-emitting device including the light-emitting element; and an electronic device and a lighting device including the light-emitting device.
2. Description of the Related Art
A light-emitting element including an electroluminescence layer (also referred to as an EL layer) has a structure in which the EL layer is sandwiched between a pair of electrodes. When voltage is applied between the pair of electrodes, the EL layer emits light. The EL layer contains an organic compound and includes at least a light-emitting layer containing a light-emitting substance. Such a light-emitting element has a simple structure. The light-emitting element has attracted attention as a next-generation flat panel display element because of its characteristics such as thinness, lightweight, high speed response, and direct-current low-voltage driving. Further, the light-emitting element is an area light source and thus is considered applicable to a light source such as a backlight of a liquid crystal display or lighting.
An emission mechanism of the above light-emitting element will be described. When voltage is applied between the pair of electrodes, electrons injected from a cathode and holes injected from an anode recombine in the EL layer. As a result of the recombination, energy is released and light is emitted.
A light-emitting element has been proposed in which, in order to increase the emission luminance of the light-emitting element, a plurality of light-emitting units (in this specification, also referred to as EL layers) are stacked and current with the same current density as in the case of a single layer is fed (e.g., Patent Document 1).
Patent Document 1 discloses a light-emitting element in which a plurality of light-emitting units are divided by charge-generation layers. Specifically, in the light-emitting element disclosed in Patent Document 1, a charge-generation layer containing vanadium pentoxide is stacked over an electron-injection layer containing an alkali metal of a first light-emitting unit, and a second light-emitting unit is stacked over the charge-generation layer. Vanadium pentoxide contained in the charge-generation layer is a kind of metal oxide.